Neuropeptides are secreted signaling molecules that function in intracellular communication in the central nervous system and the endocrine system. Neuropeptides are generated from larger, inactive polypeptide precursors called "pre-pro-proteins." These molecules are proteolytically cleaved and activated in a series of enzymatic processing steps that occur in the organelles of the secretory pathway. The first processing step takes place in the endoplasmic reticulum (ER), where the N-terminal "signal sequence," common to all secreted proteins, is removed. The remaining pro-protein is transported to the Golgi apparatus and further processed to produce the active neuropeptide(s). The neuropeptide is then secreted from the "signaling" cell in which it originated and binds to a specific receptor located on the surface of a "target" cell. The binding of the neuropeptide to its specific receptor evokes a response from the target cell, and the nature of this response depends on the particular neuropeptide, its specific receptor, and the signal transduction mechanisms of the target cell. Neuropeptides can function in the endocrine system by stimulating the secretion of hormones from target organs and by regulating gene activity in target cells. Neuropeptides can also function as neurotransmitters by regulating ion channel permeability in target cells and by innervating target tissues.
In mammals, preproneurotensin/neuromedin N (prepro-NT/NN) is processed into two active neuropeptides, neurotensin (NT) and neuromedin N (NN). These neuropeptides have been isolated from human, bovine, rodent, and canine sources and are identical in amino acid sequence among these sources. On the other hand, the prepro-NT/NN precursors from these sources are homologous, but not identical. (Kislauskis, E. et al. (1988) J. Biol. Chem. 263:4963-4968.) In the rat, prepro-NT/NN is a 169-amino acid polypeptide. The first 22 amino acids are removed to generate pro-NT/NN. NT, a tridecapeptide, and NN, a hexapeptide, are arranged in tandem at the pro-protein's C-terminus, spanning amino acids 150-162 and 142-147, respectively. Pro-NT/NN is cleaved by a pro-protein convertase, either PC1 or PC2, at Lys-Arg motifs flanking both the NT and NN amino acid sequences, thereby liberating these two neuropeptides. (Rovere, C. et al. (1996) J. Biol. Chem. 271:11368-11375.)
Studies in rat have shown that the messenger RNA (mRNA) encoding prepro-NT/NN is primarily expressed in the brain and in the intestine. The relative levels of this mRNA correlate with the levels of NT immunologically detected in these tissues; 10% of detectable NT is present in the brain, 85% is present in the intestine, and the remainder is present in other peripheral body tissues and in the circulation. (Carraway, R. and Lehman, S. E. (1976) J. Biol. Chem. 251:7045-7052.) NT functions primarily as a central neurotransmitter in the brain and as a peripheral hormone in the intestine. The function of NN is less well characterized, although its role is likely related to that of NT because both NT and NN share similarities in their amino acid sequences and bind to the same receptors. (Vincent, J. P. (1995) Cell Mol. Neurobiol. 15:501-512.)
In the rat intestine, NT is found in specific endocrine cells, the "N-cells", of the intestinal mucosa and in nerve fibers of the entero-nervous system. NT secreted from N-cells stimulates the pancreas to secrete hormones such as insulin, glucagon, and somatostatin in response to the ingestion of food. (Dolais-Kitabgi, J. et al. (1979) Endocrinology 105:256-260.) Moreover, NT modulates the contraction of rat intestinal smooth muscle preparations in vitro, suggesting that NT may function as a neurotransmitter in peripheral nervous systems as well as in the central nervous system. (Kitabgi, P. (1982) Ann. NY Acad. Sci. 400:37-55.)
In the rat brain, NT is distributed primarily in the hypothalamus, brain stem, and regions of the mid-brain. In the hypothalamus, NT stimulates the secretion of corticotropin releasing hormone, growth hormone releasing hormone, and gonadotropin releasing hormone, which in turn directly stimulate hormonal secretions from the pituitary gland. (Rostene, W. H. and Alexander, M. J. (1997) Front Neuroendocrinol. 18:115-173.) NT may function as a neurotransmitter in the neurons of the hypothalamus and midbrain by directly antagonizing the effects of another neurotransmitter, dopamine. Administration of NT directly to the mid-brain alters aspects of dopamine-mediated behaviors, such as locomotion and hunger sensation. Moreover, the effects of NT on these and other behaviors mimic the actions of anti-psychotic drugs. (Lambert, P. D. et al. (1995) Ann. NY Acad. Sci. 757:377-389.)
In humans, abnormal levels of NT have been implicated in the pathology of neurological diseases and cancer. The manifestations of Parkinson's disease are caused by lesions in the basal ganglia, a mass of nerve fibers in the mid-brain that control motor activity via dopaminergic mechanisms. Fernandez, A. et al. (1995; Peptides 16:339-346) reported that the basal ganglia from parkinsonian brains contained higher levels of neurotensin compared to that from normal basal ganglia. In addition, high affinity receptors for NT that are normally present in the basal ganglia were absent from the parkinsonian basal ganglia. (Sadoul, J. L. et al. (1984) Biochem. Biophys. Res. Comm. 125:395-404.) Furthermore, Sharma, R. P. et al. (1997; Am. J. Psychiatry 154:1019-1021) reported that the NT levels in the cerebrospinal fluid of patients with schizophrenia or related behavioral disorders were inversely correlated with the severity of the patients' psychopathology. Moreover, inappropriate expression of NT has been detected in colon and prostate cancer tissues, and several small cell lung cancer cell lines have demonstrated unusually high affinity binding of NT to concentrated receptors on their cell surfaces. (Evers, B. M. et al. (1996) Ann. Surg. 223:464-470; Kapuscinski, M. et al. (1990) J. Clin. Endocrinol. Metab. 70:100-106; Allen, A. E. et al. (1988) 9suppl1:57-61.)
Defects in the processing of pro-NT/NN have been correlated with the onset of obesity in mice with homozygous mutations in the gene encoding carboxypeptidase E, a proteolytic enzyme. (Rovere, C. et al. (1996) Endocrinology 137:2954-2958.) Levels of NT and NN in hypothalamic extracts from these mice were reduced to 80% of that in extracts from normal mice. Furthermore, unusually high levels of partially processed pro-NT/NN were present in these mutant extracts.
The discovery of a new human homolog of preproneurotensin/neuromedin N and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cell proliferative, neurological, and endocrine disorders.